Can't delete old docker images

Running Docker on OS X 10.14.6. I have several storj images (https://storj.io/) that are periodically replaced by their updater image. It doesn’t appear to be deleting the old ones. That may be because when I tried to do it manually for one of the unused images with the command “docker rmi 919733d1d7b5”, Docker complains

Error response from daemon: open /var/lib/docker/image/overlay2/.tmp-repositories.json207472833: read-only file system

So apparently somehow on the Linux VM that is provided by Docker under OS X, parts of the filesystem have become read-only.

What could cause that and how can I fix it?